The invention relates to an arrangement in a recovery boiler, into which spent liquor to be combusted and combustion air are fed, the arrangement comprising superheaters in the upper part of the recovery boiler for recovering heat.
In a recovery boiler of a pulp mill the black liquor formed during the manufacturing of pulp is combusted such that on one hand heat energy is recovered and on the other hand chemicals are recovered, which can be returned to the process taking place in the pulp mill. Heat is generated in the furnace during combustion and is recovered utilizing walls and other heat transfer surfaces. The heat transfer surfaces are connected together in different ways so that both the recovery boiler and a flue gas duct placed thereafter are provided with different heat transfer surfaces, such as boiler banks and economizers, to allow cold water to heat and further to evaporate and finally the saturated vapour is heated in the superheaters in such a manner that high-pressure vapour is superheated to meet the needs of the pulp mill and to produce electricity.
In modern recovery boilers, superheaters are placed above a so-called nose in the uppermost part of the recovery boiler in a space, through which flue gases flow from the recovery boiler to the flue gas duct. Typical superheaters are elements suspended from the upper parts thereof that comprise vertical tubes, in relation to which the flue gases flow mainly transversely.
As a theoretical starting point it is preferable that the hottest, i.e. the last superheaters in the flow direction of vapour, were placed in the hottest possible flue gases. This is not always possible, for instance owing to corrosion. However, the heat recovery capacity of modern recovery boilers should be improved from the present one and at the same time matters like corrosion should be taken into account.